1. Field of the Invention
The present invention relates to an apparatus for fabricating a glass rod and a method for the same, in particular, to an apparatus and method for fabricating a desirable diameter glass rod by feeding a relatively large diameter glass preform such as an optical fiber glass ingot into a furnace, heating the preform in the furnace and drawing the heated preform from the furnace.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2006-193397 discloses a method for fabricating a desirable diameter glass rod by measuring a diameter of a preform during drawing at a region where a deformation (diameter reduction) is progressing and a diameter at a region where the diameter reduction is almost completed, and adjusting the feed speed and drawing speed of the glass preform with respect to a furnace based on the measured diameters.
The description of known art in Japanese Patent Laid-Open No. H11-011970 (1999) discloses a method of pre-measuring a diameter of a glass preform along a longitudinal direction of the glass preform, determining a ratio between a feed speed and a drawing speed of the preform, and fabricating a constant diameter glass rod based on the ratio.
Japanese Patent Laid-Open No. 2006-219331 discloses suppression of diameter fluctuations of a drawn glass rod caused by a shifting of a reference position defining a feed speed and a drawing speed of a glass preform as drawing process progresses, by changing the amount of a feed and the reference diameter position of the glass preform.
Conventionally, the ratio of the target diameter of a glass rod with respect to the diameter of a glass preform (referred to as a diameter reduction ratio below) has been about 20% to 50% and relatively small. Accordingly, the control method of Japanese Patent Laid-Open No. 2006-193397 could suppress the diameter fluctuation to a required level. Recently, however, a larger size optical fiber preform is required, and a glass rod having a relatively small diameter deformation, in which a diameter reduction ratio is about 60% to 95%, is required. For example, when a 160 mm to 170 mm diameter glass preform is drawn into a 150 mm diameter glass rod, the diameter reduction ratio is 88% to 94%.
To implement the feedback control disclosed in Japanese Patent Laid-Open No. 2006-193397, it is necessary to measure the diameter at around a position where the diameter is substantially reduced in a diameter decreasing region. In case of a relatively large diameter reduction ratio, however, a position where the diameter is substantially reduced is adjacent the heater in a furnace. Accordingly, it is difficult to directly measure the diameter at this position. If the diameter used for the feedback control is measured at a location spaced from the heater to some extent so as to prevent an affection of the heater, the response of the feedback control could be lagged. Thus the feedback control may not be appropriately implemented. As a result, a large fluctuation can be generated in a drawn glass rod.
According to the method disclosed in Japanese Patent Laid-Open No. H11-011970 (1999), a relatively desirable diameter fluctuation value can be obtained even at a diameter reduction ratio of 60% to 95% in the case of a constantly stable diameter glass preform. The method, however, may cause an unacceptable diameter fluctuation (specifically, more than ±1%) at an end portion of a usable region in a drawn glass rod at the end of a drawing process, when a glass preform has a relatively large diameter fluctuation in a longitudinal direction thereof.
The method disclosed in Japanese Patent Laid-Open No. 2006-219331 can suppress a diameter fluctuation of a glass rod. In the method, however, a criterion for changing the reference diameter position is indefinite, and an unacceptable diameter fluctuation may be generated depending on a condition of the diameter fluctuation of the glass preform. In addition, in the embodiment of the publication, a 130 mm diameter glass preform is drawn into a 30 mm diameter glass rod, that is, the diameter reduction ratio is considerably small, such as 23%. The publication fails to disclose a method for suppressing a diameter fluctuation of a drawn glass rod in case of a relatively large diameter ratio such as 60 to 95%.